Slávek Banko composed on 2019-02-05 01:06 (UTC+0100):
First, for XAUTHORITY should be: XAUTHORITY=/var/run/xauth/A${DISPLAY}-*
This works: DISPLAY=:0 XAUTHORITY=/var/run/xauth/A${DISPLAY}-msFdGa import -window root iitdm.png
These do not DISPLAY=:0 XAUTHORITY=/var/run/xauth/A${DISPLAY}-* import -window root iitdm.png DISPLAY=:0 XAUTHORITY=/var/run/xauth/A${DISPLAY}* import -window root iitdm.png
Of course, the one that does work requires lookup of the content of /var/run/xauth/ after a reboot.
Second, I do not see in the man page that xwd would accept the environment variable XAUTHORITY or option like -auth or -xauth.
I don't know why the URL I was working from used xwd instead of import.
Another URL that fails to provide a way that works, it too is for gdm: https://www.howtoforge.com/how-to-take-a-screenshot-of-your-login-screen
Search on kde.org only found shooting desktop with Spectacle, not greeter.